TW503634B - Short code detection method - Google Patents

Abstract

A method is disclosed for receiving a transmitted signal in a communication system employing CDMA techniques wherein the transmitted signal includes a plurality of short codes, each of which is transmitted repetitively over a fixed period of time and where the received signal has CW interference in addition to the transmitted signal. The method includes using a Sequential Ratio Probability Test (SPRT) for detecting the presence of the short code in a plurality of time phases of the received signal by calculating a likelihood ratio for each phase. A likelihood ratio is a comparison of the signal's Probability Distribution Function (PDF) with a background noise PDF. The background noise PDF is calculated by combining in the RAKE the current short code with the input signal.

Description

J ^ 634

V. Description of the invention (1) Printed by the Consumer Cooperative of the Intellectual Property Bureau of the Ministry of Economics Background of the Invention Field of the Invention The present invention relates to the field of code division multiple access (CDMA) communication systems. More specifically, the present invention relates to a system for accurately detecting short codes in a communication environment, and the communication environment includes continuous wave interference. Previous Technical Description With the sharp increase in the use of wireless telecommunication systems over the last decade, the limited availability of wireless radio frequency spectrum for such systems has become an important resource. The wireless communication system using CDMA technology can provide efficient use of the available spectrum by accommodating users of more than time division multiple access (TDMA) and frequency division multiple access (FDMA) systems. In a CDMA system, all household units use the same part of the frequency spectrum for communication. In general, for each geographic area, a single base station serves multiple subscriber units. The baseband data signal of each subscriber unit is multiplied by a virtual random code sequence called a spreading code, and the spreading code has a transmission rate that is much higher than that of a poor material. As a result, user signals are extended to the entire available bandwidth. Different subscriber units are distinguished from each other by assigning a unique spreading code to each communication link. -Sometimes it is also useful to transmit short-length codes in a CDMA system. It is known to use a sequential probability ratio (fPRT) detection method in the CDMA communication system technology to detect the transmission of a short code. However, in the presence of flail wave (cw) interference, the use of the known J method can lead to a large number of false short code detections. These false detections degrade the system performance due to the extension of the two 'and short code detection. Zheng (Please read the notes on the back before filling this page) «-Pack -2: -m 丨 '-4-⑽634 ⑽634 A7

V. Description of the Invention (2) The SPRT detection method requires a background noise estimation. Background noise estimation is usually performed by applying a long virtual random spreading code to a RAKE despreader. The output of the RAKE despreader has a probability distribution function (pDF). Please refer to FIG. 1A. Curve 1 shows a typical background noise Pdf, and the background noise PDF is calculated using a long virtual random spreading code without cW interference. Curve 3 shows a typical PDF in the presence of an effectively detected signal. However, when CW interference is present during the transmission of the short code, the background noise PDF is a curve similar to that of i, where curve 2 deviates from curve 1 and appears similar to PDF of a f-effect detection signal, curve 3. Because the random short code is not applied to RAKE completely, and it starts to be associated with repetitive cW interference, the noise estimate becomes distorted. Therefore, when the curve 2 is further deviated to the curve 3 due to the existence of C w interference, the SPRT detection method turns the false detection invalid noise into a valid signal. Printed by the Consumer Cooperative of the Intellectual Property Bureau of the Ministry of Economic Affairs Please refer to Figure 1B, which shows a block diagram of a prior art short code detector system 10. The short code detector system 10 is usually located at the base station to detect short codes received from the subscriber unit. A signal including short code, continuous wave interference, and other types of background noise is applied to the short code detection system 10 ′ via the detector input line 12 and is received by the detector input block 14. The detector input block 14 contains a raKE demodulator with M different phases. RAKE Xiao Wuyi Zhou Bianyi processes the input signal by combining the input signal with a short preamble. The pilot code is a pseudo-random code, which is generated locally by the base station and transmitted by the user who initiated the call setup. The first output signal of the detector input block 14 is applied to the detection block 16 of the detector system 10. Detection block 16 includes a SPRT detection method. Detection area -5- This paper size applies to China National Standard (CNS) A4 (210 X 297)

Block 16 < The output signal appears on a decision line 20. Regardless of whether a short code exists in the signal received by the input block 14, the signal of the decision line 2Q indicates the decision of the SPRT detection method of the detection block 16. '2 input ^ The second output signal of block 14 is applied to a noise estimator. The noise A estimated state includes a separate < RAKE demodulator (Αυχ rake), and RA = E de-emitter uses _ Long virtual random code and input signal to perform a scene noise estimation. The result of the background noise estimation performed in block 18 is a PDF of the gIPRT detection method applied to detection block 16. Now refer to FIG. 2 ', which shows the short code detection method 40 of the prior art. The detection method 40 is used to detect the existence of the short code meaning transmitted to a wireless communication system. For example, the 'short code detection method 40' is applicable to the short code detector system i0i to detect block 16 to detect whether a short code exists on the input signal of the input line 12. The execution of the short code detection method 40 starts at the start block 42 and proceeds to step 44 in which one of the M different phases of the RAKE 14 is selected. The short code detection method 40 proceeds to step 46, where a background noise estimation (in the noise estimator 18 of FIG. 1B) performed by AUX RAKE is updated. The noise estimator 18 applies this signal to the detection block 16. In step 50, a signal from one of the selected phases of the input line 12 is sampled, received by the input block 14, and applied to the detection block 16 for calculation according to the short code detection method 4o. -Now refer to FIG. 3A, which shows one of the operations of the short code detection method 40. The graphic table 70. One acceptance threshold 74 and one rejection threshold 76 and two likelihood ratios 80, 84 are shown. Likelihood ratio 値 is one of the decision variables familiar to those skilled in the art. When deciding whether there is a signal in the communication system -6- This paper size is applicable to the Chinese National Standard (CNS) A4 specification (210 X 297 mm) f Read the "Precautions on the back before filling in this page). Printed A7 by the Consumer Cooperative of the Intellectual Property Bureau of the Ministry of Economic Affairs

V. Description of the invention (4) Richi 1, 1 'Likelihood ratio is very useful. The likelihood ratios 値 80,84 have a start 値 located between about two thresholds 値 74 ′ 76. The two likelihood ratios are repeated by the short code detector to 40, and compared with the thresholds 74 and 76 to determine whether a short code exists. Although the beginning of the likelihood ratio 値 80,84 is located at about the threshold of 74,76, the 'likelihood ratio 値 80,84 is adjusted, and these adjustments can be positive or negative' depends on the detection method 40 Its calculation. When the likelihood ratio 値 of a phase increases and moves in the direction of the acceptance threshold 74, the confidence level of the existence of the short code increases. When the ratio of Ran exceeds the acceptance threshold of 74, the confidence level is sufficient to determine that a short code exists at that phase. When the likelihood ratio 値 decreases and moves in the direction of the rejection threshold 値 76, the confidence level that the short code does not exist in the phase increases. When the likelihood ratio exceeds the rejection threshold of 76, the confidence level is sufficient to determine the absence of the short code. Returning to FIG. 2, the current phase likelihood ratio 値 is updated in step 54. Those skilled in the art should understand that the likelihood ratio 値 is calculated for each phase of the M different phases of RAKE. The current phase likelihood ratio 値 is calculated from the background noise estimation in step 46 and the input sampling performed in step 50. ^ Printed by the Consumer Cooperatives of the Intellectual Property Bureau of the Ministry of Economic Affairs In step 56, it is determined whether the likelihood ratios of all M phases are lower than the rejection threshold 76. Even if only one of these likelihood ratios is higher than the rejection threshold 7 6, a short code may exist in the received transmission. In this case, the execution of the short code detection method 40 proceeds to step 58. At step 58, a decision is made as to whether any of the likelihood ratios calculated by the detection method 40 is higher than the acceptance threshold 74. If any likelihood ratio 値 is higher than the acceptance threshold 如 74, the paper size determined in step 5 applies the Chinese National Standard (CNS) A4 specification (210 χ 297), 503634 A7 B7, the employee ’s intellectual property bureau ’s consumption The cooperative prints the fifth and description of the invention (5), and then determines whether a short code exists in step 60. If the detection method 40 is operated in the detection block 16 of the short code detector system 10, this decision can be displayed by the decision line 20. If all the likelihood ratios are lower than the rejection threshold 値 76, as determined in step 56, it is confirmed that k does not know that the code exists at any phase of the μ phase of the received signal. Therefore, the detection method 40 proceeds to step 52, in which the likelihood ratios of all µ-phases are cleared. The phase of the local spreading code, the pilot code, is advanced for RAKE in step 48, and the next RAKE phase is selected in step 44 | If a likelihood ratio 値 is higher than the rejection threshold 値 76, but no However, the ratio 値 is higher than the acceptance threshold 値 74. As determined in step 58, the detection method 40 advances through the path 59, thereby obtaining a new sample of the signal phase (step 50). The detection method 40 acquires and processes the duplicate branches of the new sample in this way via the path 59, resulting in the adjustment of each likelihood ratio 値 toward or away from the threshold 値 74, 70. The short code detection method 40 repeatedly advances through path 59 until any of the following conditions are met: 1) one of the likelihood ratios 超过 exceeds the acceptance threshold 値 74; or 2) all likelihood ratios 低于 are lower than the rejection Scolded the limit 76. Only when the * of these two events occurs ′ is there a sufficient level of confidence required to determine whether or not a short code exists. The number of samples required for one of the two events to occur is one of the efficiency measures of the short code detection method 40. Repeated branches through path 59 may provide increasing likelihood or decreasing likelihood of the presence of a short code. For example, in the case of the first likelihood ratio 値 80 shown in FIG. 3A, the repeated branching of the path 59 leads to the adjustment of the likelihood ratio 値 80 in the direction of the rejection threshold 76. When the continuous operation of the detection method 4 -8-

(Please read the precautions on the back before filling out this page) • ml · equipment, # 503634 A7 B7 Printed by the Consumer Cooperatives of the Intellectual Property Bureau of the Ministry of Economic Affairs 5. Description of invention (6) Can cause the likelihood ratio to exceed 80 When the open heart is limited to 76, there is a sufficiently high level of confidence to determine that no short code exists in the current phase. Repeated branches via path 59 may also provide an increasing probability of the presence of a short code. That is, in Figure 3A, the brother of 7F-likelihood ratio is expected. In the shape of LM and h, continuous sampling leads to an adjustment of the likelihood ratio 値 84 approximately along the direction of acceptance threshold 値 74. When the likelihood ratio 値 84 exceeds the acceptance threshold 値 74 through the continuous branch of path 59, there is a high enough confidence level to determine whether a short code exists in the arrows, daggers, and 9 phases.

quasi. V Figure 7 is a graph of the average number of samples required to obtain a short code using detection method 40 in the presence of acoustic CW interference. The figure shows that when the amplitude of the CW interference is greater than 0.2 times the size of the background noise, the number of samples required to obtain a short code increases sharply. When cwf ^ + ^ n A f ^ ding I is greater than 0 · 6 times the size of the background noise, the decrease in the number of samples as shown in the figure does not indicate an improvement in short code detection performance, but is reflected at this time. Test the facts that have begun to appear. As shown in FIG. 7, when a conventional SPR method is used, for example, the test method 40, the low-level CW background interference increases the short code acquisition time. In addition, the higher level of CW interference causes short codes to be falsely detected, and false detection also results in unacceptably long acquisition times required to measure a valid short code. The patentee of this patent application recognizes that a short code detection method is needed, and the pseudo code detection method can reliably and quickly detect whether a short code exists in a CDMA transmission including two w background noise. " n SUMMARY OF THE INVENTION A method for receiving ~ transmitting signals in a communication system using CDMA technology has been revealed, in which the transmitting signal contains multiple short signals, = = = (Please read the precautions on the back before filling this page) Equipment--9- V. Description of the invention (7) The code is repeated in a fixed time interval. This method is particularly useful for rejecting cW interference that can be received with the transmitted signal. This uses an SPRT to detect the multiple phases of a signal and the received signal with I & f4 ', which is based on a likelihood ratio of phase-mother-phase. For each phase of the signal being tested, the likelihood ratio is updated until it reaches any of the following two $ limits-Meets Existence-The threshold of the detected short code, and does not exist -Short code threshold limit. The likelihood ratio 値 is the probability distribution of the signal = number (PDF) compared to the background noise pDF. The pDF is calculated by passing the bluff through the rake despreader. The background noise PDFs are calculated by combining the current short preamble with the input signal in RAKE. When the preamble changes, a new background noise PDF is calculated. Brief description of the drawings FIG. 1A is a probability distribution function of a received signal and background noise; FIG. 1B is a short code detector system of the prior art; FIG. 2 is a flowchart of a short code detection method of the prior art. The code detection method is suitable for short code detection using the short code detector system of Fig. 1B; Fig. 3 A is the likelihood ratio and decision threshold for a short code detection method; "° Fig. 3 Β Is a block diagram of a short code; FIG. 4A is a preferred example of the present invention; FIG. 4B is a flowchart of a shortcode detection method of the present invention; FIG. 5 is a fake of the prior art shortcode detection method of FIG. Figure of probability of warning effectiveness; Figure 6 is a graph of probability of false warning effectiveness of the short code detection method of Figure 4; -10- This paper size applies the Chinese National Standard (CNS) A4 specification (210 X 297 mm) -I- II (Please read the notes on the back «I before completing this page) · Printed by the Consumer Cooperatives of the Intellectual Property Bureau of the Ministry of Economic Affairs 503634 A7 ^ -----_ B7_. _ V. Description of the invention (8) Figure 7 is Figure 1 A graph of the average sampling number performance of the prior art short code detection method; and FIG. 8 is a short code detection method of FIG. 4 Graph of average sample number performance. DETAILED DESCRIPTION OF THE DRAWINGS The present invention will be described below with reference to the drawings, in which like reference numerals denote similar components in all figures. Please refer to FIG. 4A, which shows the short code detection of the present invention. A block diagram of the detector system 4. A signal including short code, continuous wave interference, and other types of background noise t signal is applied to the short code detector system 400 via the contention detector input line 412, and the detector The input block 414 receives it. The detector input block 414 contains a RAKE demodulator with different phases. The first output signal of the detector input block 414 is applied to one of the detector systems 400 for detection. Block 416. Detection block 416 contains an SPRT detection method. The output signal of detection block 416 appears on a decision line 42. The signal of decision line 420 indicates the SPRT detection method of detection block 416. Make a decision whether or not a short code exists in the signal received by input block 414. The second output signal of the detector input block 414 is applied to a noise estimator 4 丨 8, and the noise estimator 4 1 8 I contains A separate RAKE demodulator (auX RAKE), and the demodulator uses the same short code transmitted by the user. The SPRT detection method of detection block 4 16 uses subject and noise estimation to more accurately detect a valid signal as explained below. The existence of the number. Please refer to FIG. 4B, which shows a short code detection method 100 according to the present invention. The short code detection method 100 can be used in the short code detector system 4 0 0 ′ shown in FIG. 4A. Detect whether a short code 8 8 a-c exists in each phase of the received signal -11- This paper size applies to China National Standard (CNS) A4 specification (210 X 297 mm) 丨 丨 ------- -«. (Please read the notes on the back before filling out this page) Printed by the Consumer Cooperative of the Intellectual Property Bureau of the Ministry of Economic Affairs A7 Printed by the Employee Cooperative of the Intellectual Property Bureau of the Ministry of Economic Affairs

503634 A7 Β7 Printed by the Consumer Cooperatives of the Intellectual Property Bureau of the Ministry of Economic Affairs 5. Description of the invention (10) Therefore, the likelihood ratio of 8 separations is calculated and saved. Step 138 determines whether the likelihood ratios of all M phases are below the rejection threshold 値%. If the decision 138 is negative, a short code may exist in at least one of the% phases. In this case, 144 further decides whether any of these likelihood ratios is higher than the acceptance threshold (74). If the decision 144 is affirmative, a short code exists, and the execution of the short code detection method 100 proceeds to step 152 to show that the pilot signal has been acquired. If all likelihood thresholds are lower than the rejection threshold 76, as determined by the steps, the dream has a level of confidence to determine that no short code exists in any phase of the current M phase. In these cases, the detection method ιο proceeds to step 134 via branch 140. At step 134, a decision is made as to whether a three second time period has expired. In the 3 leap second time interval of the decision step 134, the change with the short code used by the user is synchronized to obtain the pilot signal. The present invention uses a time interval of 3 leap seconds as an example. Those skilled in the art should understand that the time interval for obtaining the pilot signal with the new short code is the same as the time interval for updating the background noise according to the original method. The specific time interval is not the ^ key of the present invention. If the 3 leap second timer has not expired, as determined by decision step 34, the detection continues by using the same background noise estimate. In these cases, the execution of the short code detection method 100 proceeds directly to step 20, where all likelihood ratios of the first M phases are cleared. Then, in step ", the code phase is advanced and the M new phase is processed. Therefore, step 108 and the short code detection method 100 are repeated. Change the nature of the change, and the situation is 112 steps (Please read the precautions on the back before filling this page) 襞 Order 13-}--

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I. Printed by the Consumer Cooperative of the Intellectual Property Bureau of the Ministry of Economic Affairs. 5. If the 3 millisecond time interval has expired at the time of the invention description (11), resetting is calculated as determined in step 134, and one of the background noise estimates is quickly updated. It is executed, as shown in step 132. The background noise estimation is performed by using the new short code ^ for the method described in step 104. The time interval of 3 milliseconds coincides with the use of a new short code. ^, Refer to FIG. 3B again, because each short code 88a-c has a corresponding update interval 92a_c at the beginning of using a new short 88a-c foot, so it is stated in the background of | Step 132, the update is estimated in the short Correspondence between the codes in the update interval 92a-c is executed. The sampling at step 132 should be taken very quickly after the time period has expired. In the preferred embodiment of the present invention, the sampling is obtained in some symbol intervals used by the new short codes 88a-c. This original method for updating background noise results in the implementation of the short code detection method 100 which operates on a set of samples, and the set of samples has a noise estimate using the same ~ sampled short code time slot. Using noise estimates from the same time slot as the sample can improve the accuracy of the short code detection method 100. The background noise estimation is used to update a background noise pDF in step 124. At step 12 all likelihood ratios are doubled to clear. The local code phase is received in step 112, and a new phase and a new sample are processed, so the steps are repeated and the short code detection method 100 is started again. Please refer to FIG. 4B again. If a likelihood ratio 値 is higher than the rejection threshold 値 76 but no likelihood ratio 値 is higher than the acceptance threshold 値 74, as determined in step 144, the execution of the short code detection method 100 is performed through a branch. 15〇 Proceed to step 148 At step 148 'decide whether the 3 millisecond time interval has expired. If the time interval of 3 milliseconds has not expired, the detection method 100 will continue to operate at the current time. 108 seconds before the message, please read the note on the back first.

Filling page • 14- This paper size applies to China National Standard (CNS) A4 (210 X 297 public love) A7 ^^ -----_— V. Description of the invention (12) ~~ " ^ f 杂The information is estimated, and samples of these "phases of each phase" are obtained at step 116. If the 3 second time interval has expired, this indicates a new shame 1 ^ is again used. Therefore, in block 丨In 46, the timer is reset, and the fast update of the same π noise is double-executed. In step 42, the background noise estimation is adjusted, and in step 116, a new sample of each phase is obtained. It is stated that in each pass of the detection method, the time interval of 3 seconds is doubled to test V. No matter whether the execution of the detection method 100 passes the branch 14, the likelihood ratios of all items f exceed 1 Rejection threshold, or when execution passes branch 150, none of the current likelihood ratios have exceeded the acceptance threshold. 0 Please refer to Figure 180 in Figure 5, Figure 18 states that under multiple cw sizes , One of the short code detection methods of the technology is 40% false acquisition probability. When the CW interference is 0.5 times the background noise When normalizing the frame, the false acquisition probability of the short code detection method 40 of the prior art began to rise sharply, and when cw interference 疋 0.8 multiplied by the background noise, the false acquisition probability reached 100%.-Ministry of Economy Intelligence The property bureau employee's consumer cooperation is loosely printed, but 'please refer to FIG. 6, and the second figure 200 describes that the original short code detection method has a false acquisition probability of 100 under the size of multiple continuous waves. As shown in the figure It is shown that, even when the CW interference is 4 times as large as the background noise, the short code detection method 100 lacks false acquisition probability is substantially zero. Therefore, compared with the prior art short code detection method 40, this The invention provides a dramatic improvement in the performance. Now please refer to Figures 7 and 8. The second figure 220, 240 is shown, and it is -15- ft i applicable ¥ and home standard grid coffee x 297 public love) ---- 503634 A7 B7 V. Description of the invention (13) The method of detecting the short code by the consumer cooperation loose description of the Intellectual Property Bureau of the Ministry of Economic Affairs 40,100 determines whether it exists-the average number of samples required for short codes. Those familiar with this technical field It should be understood to determine the number of samples required The less the better the method is. When the continuous wave interference increases, the short code detection method 40 of the prior art requires far more samples to detect a short code. As shown in Figure 7, when C w interference When the size increases, the average number of samples can increase by an order of magnitude. For CW interference greater than 0.6 times the size of the background noise, the decrease in the number of samples shown in the graph 220 does not mean that the short code detection performance is good. Instead, it reflects the fact that false detection starts to appear at this time. T In contrast, as shown in Figure 8, the average number of samples required by the original detection method 100 is in a large range of continuous wave sizes. Substantially maintained. In addition, for some CW interferences, the number of samples required for detection method 100 remains much lower, and these CW interferences are much higher than the CW interferences that caused the number of samples in the previous detection method 40 to increase dramatically. The essence of the present invention is to eliminate false display of short codes. The foregoing description of the preferred embodiments is provided to enable any person skilled in the art to make and use the present invention. Those familiar with the technical field should be able to easily propose various modifications of the examples, and the general principles defined herein can be applied to other examples without providing an original contribution. Therefore, this article does not mention that the present invention is limited to the examples shown, but that the present invention is applicable to the widest range that conforms to the unveiling of plants, principles, and characteristics. Can be displayed (please read the precautions on the back before filling out this page). ¾. -16- This paper size applies to China National Standard (CNS) A4 (210 x 297 mm)

Claims (1)

503634 A8 B8 C8 D8 6. Scope of patent application i — A method for receiving a transmission signal from one communication unit in a communication system, wherein the transmission signal includes multiple short codes, and the communication unit repeatedly transmits at least one short code, The short code is periodically updated, and the method includes the following steps: (a) using the same short code transmitted by the communication unit to obtain a background noise estimate; (b) using the background noise estimate to adjust a Likelihood ratio 値; (c) Compare the likelihood ratio 値 with a predetermined threshold 値 to determine whether the likelihood ratio 値 exceeds the predetermined threshold 値; (d) repeat step (a)- (c) Until the likelihood ratio 値 exceeds the predetermined threshold 値. 2. The method according to item 1 of the scope of patent application, further comprising the following steps: comparing the likelihood ratio 多个 with a plurality of predetermined thresholds, so at least one of the thresholds is an acceptance threshold; And at least one of the thresholds is a rejection of thresholds. 3. If the method of claim 2 of the patent application, and therefore the transmission signal has multiple signal phases, the method further includes the following steps; comparing multiple likelihood ratios corresponding to the multiple signal phases to the multiple priors The threshold of the decision is rampant. 4. If the method of claim 2 of the patent application scope further includes the following steps: if one of the plurality of likelihood ratios 似 exceeds one of the plurality of predetermined ratios 値, the forward signal Phase. 5. If the method of applying for the fourth item of the patent scope, further includes the following steps: ^ __ -17- This paper size applies to China National Standard (CNS) A4 (210 X 297 public love) (Please read the precautions on the back before reading) (Fill in this page) Printed by the Consumer Cooperatives of the Intellectual Property Bureau of the Ministry of Economy 503634 A8 B8 C8 D8 Scope of patent application If one of the multiple likelihood ratios 値 exceeds the rejection threshold 値, the signal phase is advanced. 6. The method according to the scope of the patent application, further comprising a step of τ: sampling the transmitted signal to adjust the likelihood ratio 根据 according to the sampling. 7. The method of claim 6 in which the communication system includes a receiver having a RAKE, and the method further includes the following steps: Adjusting the likelihood ratio 根据 based on a sample obtained from the RAKE. 8 · If you apply for the method of item k in the patent, the transmitted signal contains multiple time slots (., · These time slots are separated by multiple time slot boundaries and each time slot contains a time slot update interval, This obtaining step is performed at the update interval = 9. If the method of item 8 of the patent scope is declared, the time slot update interval substantially occurs immediately after the time slot boundary. '10. The method of item 9 further includes the following steps: Adjust the likelihood ratio 在 一 in a selected time slot based on only one noise estimate obtained in the update interval. 11. If the method of item 2 of the gas range is applied, further Including the following steps: When the likelihood ratio 値 exceeds the acceptance ratio 値, determine whether there is a short code 0 12. — A system receiving a transmission signal from one of the communication units in a communication system, wherein the transmission signal Contains multiple short codes, and the communication unit repeatedly transmits at least one short code. The short code is periodically updated, and the system includes: a device to obtain a back using the same short code transmitted by the communication unit. This paper size applies to China National Standard (CNS) A4 (210 X 297 mm) ------------ * 11 (Please read the precautions on the back before filling this page) tr --- ------ #-Printed by the Intellectual Property Bureau of the Ministry of Economic Affairs, Consumer Cooperatives 503634 88859 ABCD VI. Patent Noise Estimate for Scenery; Device to use this background noise estimate to adjust a likelihood ratio based on the transmitted signal 値; And the device to compare the likelihood ratio with a predetermined threshold to determine whether the likelihood ratio exceeds the predetermined threshold. Printed by the Consumer Finance Agency, Intellectual Property Bureau, Ministry of Economic Affairs- 1 9-(Please read the notes on the back before filling this page) This paper size applies to China National Standard (CNS) A4 (210 X 297 mm)